Process for routing data packets in a mobile node network and associated terminal

ABSTRACT

A method of routing data packets (P) between a source node (S) and a target node (C) in an ad hoc network comprising mobile nodes ( 1 - 14 ) that can be located and moving on traffic routes (L 1 , L 2 , L 3 , C 1 , C 2 , C 3 ) of a particular geographical network ( 10 ) forming between them a plurality of intersections (I 1 -I 6 ). This method comprises a destination intersection selection step in which a carrier node (S) of the packets (P) selects a destination intersection ( 12 ) from neighbor intersections and according to traffic conditions; a step of seeking one or more neighbor mobile nodes ( 1 ) of the packet carrier node nearer the selected destination intersection ( 12 ) than the carrier node; and, if one or more neighbor mobile nodes ( 1 ) are found, a step of transferring the packets (P) from the packet carrier node (S) to the neighbor node ( 1 ) that has been found, so as to route the data packets to the selected destination intersection.

RELATED APPLICATIONS

This is a U.S. National Phase Application under 35 USC §371 ofInternational Application PCT/FR2007/051279, filed on May 15, 2007.

This application claims the priority of French application no. 06/04737filed on May 24, 2006, and the content of which is hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

The field of the invention is that of ad hoc communications networks.

Ad hoc communications networks use the radio medium. They consist ofmobile and/or fixed nodes that have the property of automatically anddynamically constituting a network capable of routing packets from anypoint of the network to any other point of the network after radiocommunication is set up between a node and its neighbors.

In an ad hoc network, packets are transmitted between the source nodeand the destination node either directly if the destination node is inthe connectivity area of the source node or via intermediate neighbornodes if the destination node is out of range of the source node.

Consequently, ad hoc networks enable instantaneous deployment ofcommunications networks with no pre-existing infrastructure and nocentralized control. The network is formed dynamically, all managementtasks being divided between all nodes of the network.

The main feature of ad hoc networks is that the nodes of the networkserve or can serve as routers. The nodes themselves are thereforeresponsible for setting up and maintaining continuous networkconnectivity using specific routing protocols that enable exchange ofrouting information between neighbor nodes and enable the nodes of thenetwork to calculate communication paths to all the other nodes. Theserouting protocols send messages periodically to update the topology ofthe ad hoc network, i.e. to identify the nodes and the links betweenthem.

To be more precise, the invention relates to a routing method oftransmitting data packets between a source node and a target node of anad hoc network formed by mobile nodes moving on traffic routes of aparticular geographical network between which intersections are formed,the nodes being used as carriers for routing data packets to the targetnode.

The invention finds a special but non-limiting use in vehicular ad hocnetworks (VANET) and in urban environments.

Urban vehicular ad hoc networks are distributed and flexibleintervehicular communication (IVC) wireless communications systems inwhich the communication nodes are vehicles moving along routes of a roadnetwork of known structure.

In such systems, each vehicle is equipped with short-range wirelesstransmission means enabling it to send and receive radio-frequencysignals and thus to form, in conjunction with other vehicles, atemporary communications network.

In these networks, data packets are routed from a source vehicle to atarget vehicle using a routing protocol whereby the data packets aresuccessively forwarded by one or more vehicles moving on the routes ofthe road network between the source vehicle and the target vehicle.

Most existing routing protocols are based on geographical routing thatexploits local information concerning the explicit geographical positionof the nodes of the network in order to take decisions regardingtransfer of data packets.

A first routing protocol known as the GSR (Geographic Source Routing)protocol bases routing on the geographical position of the nodes of thead hoc network correlated with information relating to the topology ofthe network.

According to the GSR protocol, a source vehicle seeking to send a datapacket to a target vehicle calculates the shortest routing path to reachthe target vehicle based on geographical information from a road map.Note that the routing path in question is calculated in its entirety,for example using the Djikstra algorithm.

On the basis of the calculated routing path, the source vehicle thenselects a sequence of intersections through which the data packet mustpass in transit to reach the target vehicle. This set of intersectionsconsists of a set of fixed geographical points through which the datapacket is to pass.

Thus a complete sequence of fixed geographical points must be calculatedbefore sending each packet to a target vehicle, this sequence then beinginserted into the header of a packet to be sent to the target vehicle.

A drawback of the GSR protocol is that the sequence of intersectionsselected can include roads on which there are too few vehicles toprovide a good connection, which has the harmful effect of increasingthe packet loss rate.

The insertion of this kind of sequence represents an additional load onthe ad hoc network, given that it must be inserted into the header ofevery packet sent to a target vehicle. Consequently, this kind ofapproach is disadvantageous in that it cannot optimize the bandwidthallocated to the ad hoc network.

Moreover, it should be noted that the step of determining the entirerouting path for reaching the target vehicle is a greedy operation interms of computation resources. This operation being carried out only atthe source node, the GSR protocol does not provide for tracking theevolution of a moving target vehicle.

This can induce a relatively long latency time between a source vehiclesending a packet and a target vehicle receiving it.

When the routing path has been established, the data packet is forwardedsuccessively from node to node along that path using a forwardingstrategy known as “Greedy Forwarding”.

According to this forwarding strategy, a carrier node of a packet alwaysseeks to forward that packet to a neighbor node that is nearer thetarget vehicle than the carrier node itself. As soon as the data packetis received, the neighbor node in turn becomes a carrier node that inturn seeks to forward the packet to a neighbor node, and so on.

However, this forwarding strategy can fail in the particular situationwhere no neighbor node of the carrier node is nearer the target nodethan the carrier node itself. This situation is known as a “localoptimum” situation and requires a recovery solution.

The A-STAR (Anchor-based Street and Traffic Aware Routing) protocol isspecifically designed for intervehicular communications networks inurban environments and differs from the GSR protocol in that it takesroad traffic data into account when it calculates a routing path. Thistraffic data is static (for example based on statistics), however, andis not suitable for an urban network in which road traffic is constantlyevolving over time. In particular, existing routing protocols in thecontext of intervehicular communication do not at present take intoaccount space and time variations in road traffic density or thepresence of multidirectional routes.

SUMMARY OF THE INVENTION

One aspect of the present invention is directed to a method of routingone or more data packets between a source node and a target node in anad hoc network comprising a plurality of mobile nodes moving on trafficroutes of a particular geographical network forming between them aplurality of intersections. This method includes:

-   -   a step of a carrier node of a data packet to be routed to the        target node selecting a destination intersection from candidate        neighbor intersections as a function of traffic conditions;    -   a step of seeking one or more neighbor mobile nodes of the        carrier node of the data packet nearer the selected destination        intersection than the carrier node;    -   if one or more neighbor mobile nodes is found, a step of        transferring the data packet from the carrier node of the data        packet to said neighbor node that has been found so as to route        said data packet toward the selected destination intersection.

The routing method of the invention progressively routes data packets bysuccessive transfer from node to node toward the target node, each nodeselecting an intermediate destination intersection through which thedata packets must pass in transit. Thus the route taken by the datapackets across the particular geographical network is determinedprogressively, enabling virtually instantaneous tracking of changes tothe topology of ad hoc networks in which mobile nodes are constantly onthe move.

By determining the route progressively, the method of the inventionenables packets to be transmitted over much greater distances thanexisting routing protocols. The greater the distance between the sourcenode and the target node, the greater the transmission time betweenthose two nodes. The greater the transmission time, the higher theprobability of changes to the topography of the ad hoc network. Withoutfrequent calculation of the routing path, the rate of loss of packets tobe transmitted to the target node quickly reaches a high value, as inexisting protocols.

Progressive routing based on local routing information reduces the sizeof the headers of the data packets used to route the data, in contrastto routing protocols that maintain up-to-date global geographicallocation information and calculate an entire routing path.

This makes very efficient use of the resources of the ad hoc network, inparticular minimizing the bandwidth allocated to the network.

What is more, the routing method of the invention enables data packetsto be routed from a source node to a target node with a minimumend-to-end delay combined with a low data packet loss rate. Note thatthe target node can be fixed (for example a reception point in a servicestation) or mobile (for example a moving vehicle).

The routing method of the invention is therefore particularly suitablefor ad hoc networks in which the nodes are constantly moving fast,changes of network topology are frequent, and the durations of theconnections between mobile nodes are short (especially on multihoppaths).

Moreover, by taking account of traffic conditions, it limits the risk ofinterruption of packet routing caused by insufficient traffic on aselected traffic route.

According to one feature of the method of the invention the step ofselecting a new destination intersection is executed if the carrier nodeof the data packet is located at the selected destination intersection,referred to as the current intersection.

This optimizes the calculation load on the nodes by limiting executionof this step to nodes that have come into the vicinity of the currentdestination intersection.

According to one feature of the method of the invention, during theselection step the carrier mobile node of the data packets selects adestination intersection from the candidate intersections as a functionof one or more real time traffic conditions on a traffic routeconnecting the candidate intersection to the current intersection.

Taking account of real time mobile node traffic conditions makes itcertain that a traffic route between the intersection at which themobile node is currently located and the candidate destinationintersection will be selected and minimizes connectivity problems,thereby reducing routing latency and the packet loss rate.

For example, the denser the node traffic on a traffic route, the higherthe level of connectivity thereon and, consequently, the greater thenumber of packets in transit on this route likely to reach thedestination intersection in a minimum time.

According to another feature of the invention, the carrier mobile nodeselects the destination intersection from the candidate intersections asa function of the proximity of the candidate intersection to the targetnode.

The carrier mobile node therefore selects the destination intersectionthat is geographically nearest the target node and in the direction ofwhich traffic conditions are optimum.

According to another feature of the invention, each node maintains up todate at least one of the parameters belonging to the group comprisingthe position, speed and direction of movement of its near neighbornodes.

According to another feature of the invention, the carrier node selectsa neighbor mobile node as a function of the speed of the neighbor nodeand/or its progress toward the selected destination intersection.

By taking account of the speed vector (amplitude and direction) and theposition of each of its neighbor nodes, a carrier node determines theneighbor node that is nearest the destination intersection by means ofan estimation calculation. For example, the neighbor node selected bythe carrier node is that nearest the destination intersection and movingtoward it at the highest speed.

According to another feature of the invention, each node maintainsup-to-date measurements relating to the position, speed and direction ofmovement of its neighbor nodes and the step of seeking one or moreneighbor mobile nodes of the carrier node of the data packet nearer theselected destination intersection than the carrier node includes aprediction step in which the current position of a neighbor node ispredicted at a current time from these measurements.

According to another feature of the invention, the carrier mobile nodeof the data packet selects from the candidate intersections thedestination intersection having a maximum score Sj, that score beingcalculated using the following formula:Sj=α×f(T _(ij))+β×g(D _(j)), where

T_(ij) represents the traffic density of nodes between the currentintersection and the destination intersection;

-   -   D_(j) represents the distance along the curve of the routing        path that connects the destination intersection to the target        node;    -   α and β represent correction factors;    -   f is a density function of the road traffic such that        0≦f(T_(ij))≦1; and

${{g\left( D_{j} \right)} = {1 - \frac{D_{j}}{D_{i}}}},$such that −1≦g(Dj)≦1, where Di is the distance between the currentintersection and the target node.

Another aspect of the invention is directed to a communications terminaladapted to be used by a mobile node of an ad hoc network for routingdata packets to a target node of the ad hoc network, said ad hoc networkcomprising a plurality of mobile nodes moving on traffic routes of aparticular geographical network forming between them a plurality ofintersections. The terminal of the invention includes:

-   -   means for selecting a destination intersection from neighbor        intersections as a function of traffic conditions;    -   means for seeking a neighbor mobile node nearer the selected        destination intersection than said mobile node;    -   means for transferring the data packets to the neighbor mobile        node that has been found if the result of the search is        positive.

The particular embodiments and advantages of this terminal are the sameas those of the method of the invention described above.

According to another feature of the invention, the terminal of theinvention includes means for selecting the destination intersection as afunction of its proximity to the target node.

According to another feature of the invention, the terminal of theinvention includes means for keeping up to date positions, speeds anddirections of movement of its near neighbor nodes.

According to another feature of the invention, the terminal of theinvention includes means for selecting a neighbor mobile node as afunction of the speed of that node and/or its progress toward theselected destination intersection.

The invention is also directed to a wireless communications systemcomprising mobile nodes interconnected in an ad hoc structure and movingon traffic routes of a particular geographical network forming betweenthem intersections, in which system the nodes are equipped with acommunications terminal as described above.

In such systems, a data packet to be routed to a target node isprogressively routed toward that target node by successive node-to-nodehops on traffic routes on which there are sufficient mobile nodes toprovide adequate connectivity.

Alternatively, the various steps of the data packet routing methoddescribed above are determined by instructions of computer programs.

Consequently, another aspect of the invention is directed to a computerprogram including instructions for executing the steps of the datapacket routing method described above when that program is executed by acomputer.

This program can use any programming language and take the form ofsource code, object code or an intermediate code between source code andobject code, such as a partially compiled form, or any other desirableform.

Another aspect of the invention is directed to a computer-readablestorage medium storing a computer program including instructions forexecuting the steps of the data packet routing method described above.

The information medium can be any entity or device capable of storingthe program. For example, the medium can include storage means, such asa ROM, for example a CD ROM or a microelectronic circuit ROM, ormagnetic storage means, for example a diskette (floppy disk) or a harddisk.

Moreover, the information medium can be a transmissible medium such asan electrical or optical signal, which can be routed via an electricalor optical cable, by radio or by other means. The program of theinvention can in particular be downloaded over an Internet-type network.

Alternatively, the information medium can be an integrated circuit intowhich the program is incorporated, the circuit being adapted to executethe data packet routing method of the invention or to be used in itsexecution.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention emerge from thefollowing description with reference to the appended drawings, whichshow one non-limiting embodiment of the invention and in which:

FIG. 1 is a diagram showing a road network in which vehicles of an adhoc network circulate;

FIG. 2 is a flowchart showing one iteration of one embodiment of a datapacket routing method of the invention;

FIG. 3 is a flowchart showing the steps of one embodiment of theinvention that select a destination intersection;

FIG. 4 is a diagram showing one example of selecting a destinationintersection in a particular configuration of an urban intervehicular adhoc network;

FIGS. 5A and 5B are diagrams showing the forwarding of data packets Pbetween two successive intersections in one embodiment of the invention;and

FIGS. 6A and 6B are diagrams showing one example of use in oneembodiment of the invention of a recovery strategy in the presence of alocal optimum situation.

DETAILED DESCRIPTION OF ONE EMBODIMENT

The invention is described in detail below in the context of anintervehicular ad hoc network intended in particular to providehigh-connectivity services, such as web browsing, instant messaging,file sharing and real-time gaming. The protocol of the invention isequally suitable for applications such as road safety or cooperativedriving.

FIG. 1 is a diagram showing an ad hoc network comprising a plurality ofvehicles 1 to 14 moving on traffic routes L1, L2, L3, C1, C2 of aparticular road network 10, a source vehicle S sending data packets P,and a target vehicle for which the data packet P is intended.

In the FIG. 1 road map the road network 10 includes “horizontal” trafficroutes L1, L2, L3 and “vertical” traffic routes C1, C2, these routes L1,L2, L3, C1, C2 defining a plurality of intersections I1, I2, I3, I4, I5.In this example, for reasons of simplicity, traffic routes that cross atright angles are shown. However, the invention applies withoutdistinction to any traffic plan configuration including in particulartraffic routes at any angle to each other.

Each vehicle of the ad hoc network is equipped with a geographicalpositioning device, for example a GPS (Global Positioning System)receiver coupled to a GLS (Grid Location Service) so that each vehicleof the ad hoc network can be located. The GPS receiver enables a vehicleto determine its own geographical position while the GLS serviceprovides the instantaneous geographical position of the other vehiclesand in particular that of the target vehicle C.

Moreover, each vehicle is adapted to determine the position of nearneighbor intersections on the basis of location information combinedwith predefined road maps. For this purpose, each vehicle can beequipped with a standard onboard navigation device containing prestored(preloaded) digital road maps relating to the road network 10.

Each vehicle has information regarding the road traffic around each ofthese neighbor intersections. For example, this information can beprovided by a beacon message transmitted by each vehicle to itsneighbors periodically (for example every second). Alternatively, roadtraffic sensors (not shown) installed at each intersection could provideany vehicle in the vicinity of the intersection with information on roadtraffic conditions.

FIG. 2 is a flowchart showing one iteration of one embodiment of a datapacket routing method of the invention.

Firstly, there are two different situations with regard to the initialstep of the routing method.

In a first situation, the source vehicle S is seeking to send datapackets P to the target vehicle C during an initial step E0.

In the second situation, the data packets P have already been sent bythe source vehicle S and received by a vehicle referred to below as the“carrier vehicle” during an initialization step E1.

In the first situation, the source vehicle S selects during a selectionstep E3 a destination intersection to which the data packets P will berouted. This selection step E3 is described in detail below withreference to FIG. 3.

As soon as a destination intersection has been selected, the datapackets P are forwarded toward that destination intersection, asdescribed in relation to the subsequent steps of the method. The sourcenode is referred to below as the carrier node of the data packet.

Thus the data packets P are marked with the geographical position of theselected destination intersection, for example in their header. Eachvehicle of the ad hoc network keeps an up-to-date “neighbor table” whichstores information relating to its near neighbors, such as theirposition, speed (amplitude) and direction of movement at a given time.These neighbor tables can be updated by each vehicle sending a beaconmessage to its neighbors periodically, for example. Thus each vehicle ofthe ad hoc network 10 sends a beacon message to all its neighbors atregular time intervals Δt (for example Δt=1 s).

The term “neighbor” refers to a vehicle that is within radio range ofthe carrier vehicle of the data packet P. The term “near neighbor”refers to a neighbor vehicle that is at a sufficiently short distancefrom the carrier vehicle to ensure the required transmission quality orquality of service (for example by complying with a predeterminedsignal-to-noise ratio threshold parameter).

The next step is a step E7 of seeking near neighbors.

In the second situation, i.e. if the data packets P have already beensent by the source vehicle S and received by a carrier vehicle duringthe initialization step E1, a step E5 verifies whether the carriervehicle is located at the destination intersection. The expression“located at the destination intersection” means that the carrier vehicleis located within a predefined area encompassing the destinationintersection, for example a circle of predetermined radius having thedestination intersection for its center.

If this is not so, i.e. if the carrier vehicle is not located at thedestination intersection, the next step is the search step E7.

If it is so, i.e. if the carrier vehicle is located at the destinationintersection, the carrier vehicle selects during a selection step E3 adestination intersection to which the data packets P will be routed, andthe data packets P are forwarded toward that destination intersection.The next step is then the search step E7.

The search step E7 seeks neighbors of the carrier vehicle of the packetsP to be forwarded. The above-mentioned neighbor tables can be updatedduring this step.

Then, during a prediction step E9, the position of the neighbor vehiclesat the current time t2 after the time t1 is estimated from informationmost recently stored at time t1 in the above-mentioned neighbor tables(speed, direction of movement and last known position at time t1). Thispredicts the current position of the neighbor vehicles detected duringthe search step E7, i.e. their position at the current time t2.

For this purpose it is assumed that each of the vehicles is moving at aconstant speed equal to that previously detected and stored at the timet1, for example.

The steps E7 and E9 then seek at least one mobile node 2, 3 that is aneighbor of the carrier node 1 of the data packet P and nearer theselected destination intersection than the carrier node.

A test step E11 determines, on the basis of the positions estimatedduring the preceding prediction step E9, whether there is at least oneneighbor vehicle whose distance from the destination intersection at thetime t2 is less than the distance between the carrier vehicle and thesame intersection, to which the carrier vehicle can then transfer itspackets P.

If it is estimated that two or more of the vehicles would be locatednearer the destination intersection than the carrier vehicle itself atthe time t2, then the vehicle for which this distance is the shorter orshortest is selected.

If such a neighbor vehicle exists, the carrier vehicle sends the datapackets P to it in a transmission step E13, thereby marking the end ofone iteration of the method of the invention (step E15).

Otherwise, i.e. if there is no neighbor vehicle that would be nearer thedestination intersection than the carrier vehicle, then a test step E17determines whether the carrier vehicle of the packet P is sufficientlynear the destination intersection.

A reference travel time upper limit corresponding to a fixed delay isdefined for this purpose, for example, and is compared to the estimatedtravel time of the carrier vehicle to reach the destinationintersection. This upper limit is chosen to produce a delay that isacceptable given the required quality of service.

If it is estimated that the carrier vehicle is sufficiently near thedestination intersection, the vehicle carries the packet P to thedestination intersection in a step E19.

An iteration of the routing method of the invention ends as soon as thecarrier vehicle of the packet P reaches the selected destinationintersection. The next step is then the step E15.

Otherwise, i.e. if it is estimated that the carrier vehicle of the datapacket P is not sufficiently near the destination intersectionconcerned, the vehicle carries the packet in the direction of thatintersection in a step E21:

-   -   until it reaches the destination intersection; or    -   until another vehicle nearer the destination intersection enters        its transmission field/domain.

The next step is then the step E15.

In the step E15, a new iteration begins from the step E1.

An illustrative embodiment of the routing method of the inventiondescribed with reference to FIG. 2 is described below with reference toFIG. 1.

A source vehicle S moving on a traffic route L2 is seeking to send apacket P to a target vehicle C moving on a traffic route C2. For thispurpose, it is assumed that it sends the packet P to a first vehicle 1located at a first intersection I1, which is the intersection nearestthe source vehicle S. Obviously, it is also assumed that the firstvehicle 1 is within radio range of the source vehicle S.

The first vehicle 1 receives the packet P in an initial step E1. Locatedat an intersection I1 called the first intersection (positive responseto the test E5), it selects a destination intersection in the selectionstep E3.

In this embodiment, the first vehicle 1 selects as the destinationintersection a second intersection I2 because it is nearer the targetvehicle C and there is dense road traffic between the first intersectionI1 and the second intersection I2.

After selecting the second intersection I2 as the destinationintersection, the first vehicle 1 marks the packet P with the positionof the second intersection I2. The first vehicle 1 then transfers thedata packet P to a second vehicle 2 in a step E13.

For this purpose, it detects its nearest neighbors in a search step E7.In this embodiment, it detects the presence of a second moving vehicle 2and a third moving vehicle 3, both these vehicles being sufficientlynear the first vehicle 1 to receive an electromagnetic signal carryingthe data to be transmitted sent by the vehicle 1.

In the prediction step E9, the first vehicle 1 calculates the positionsof the second vehicle 2 and the third vehicle 3 at a later time t2 fromposition and speed information relating to the two vehicles 2, 3contained in its neighbor table, which is kept up to date. This exampleassumed that the third vehicle 3 is moving more slowly than the secondvehicle 2.

In a test step E11, the first vehicle 1 decides to send its packet P tothe second vehicle 2 even though, according to information contained inits neighbor table, the vehicle 2 is farther from the secondintersection I2 than the third vehicle 3. This choice is motivated bythe fact that the third vehicle 3 is moving toward the secondintersection I2 more slowly than the second vehicle 2 and thatconsequently it is highly probable that, after their current positionsare predicted, the second vehicle 2 will be nearer the secondintersection I2 than the third vehicle 3.

After sending the packet P in a step E13, a first iteration of themethod of the invention ends (step E15). The method is then iterated foreach vehicle forwarding the data packet P.

The packet P is received by the second vehicle 2 in the initial step E1which initiates a new iteration of the method described above, nowapplying to the second vehicle 2. At the end of this iteration, thesecond vehicle 2 transfers the packet P to a fourth vehicle 4 at thesecond intersection I2. Being located at an intersection, the fourthvehicle 4 selects in the selection step E3 a new destinationintersection (fifth intersection I5) and in the step E13 transfers thepacket P to a fifth vehicle 5 moving toward the target vehicle C on thetraffic route C2.

In this example, the fifth vehicle 5 is the only vehicle between thefourth intersection I4 and the fifth intersection I5 moving toward thelatter intersection I5. Consequently, the second vehicle 2 does notdetect a nearest neighbor that is nearer the fifth intersection I5 thanthe fifth vehicle 5.

In other words, the situation is a “local optimum” situation.Consequently, the recovery strategy of the invention is applied in arecovery phase P3 comprising the steps E17, E19 or E17, E21 alreadydescribed. Accordingly, in the step E21, the fifth vehicle 5 carries thepacket P as far as the fifth intersection I5.

Finally, it is assumed that when the fifth vehicle 5 reaches the fifthintersection I5 it is in radio range of the target vehicle C and so cantransmit the packet P to it directly, with no further forwarding.

FIG. 3 is a flowchart showing the steps executed to enable a carriervehicle of a data packet P to select a destination intersection.

In an initial step E30, the carrier vehicle of the packet P reaches anintersection Ii below referred to as the “current intersection”.

During a next step E32, the vehicle determines the position of nearneighbor intersections. This determination step E32 is based ongeographical information from a digital road map preloaded into astandard onboard navigation device, for example, taking account of theinstantaneous position of the vehicle in question and the position ofthe target vehicle C.

For each located neighbor intersection Ij, there is executed a phase P1of calculating a score Sj described below and used to select thedestination intersection to which the packet P must be routed.

Accordingly, the calculation phase P1 is iterated as many times as thenumber of neighbor intersections detected. In this example, the score Sjcalculated for each candidate destination intersection Ij depends on twoparameters in particular: a road traffic density and a geometricaldistance.

The calculation phase P1 includes a first step E34 of obtaininginformation relating to the existing road traffic on the traffic routeconnecting the current intersection Ii to a neighbor destinationintersection Ij. This information consists of the real time road trafficdensity on this route, constituting a first parameter to be taken intoaccount in calculating the score Sj of a candidate destinationintersection Ij, this parameter being denoted T_(ij) below.

In a second step E36, the distance between the target vehicle C and thecandidate destination intersection Ij is determined. This distance,denoted Dj below, constitutes a second parameter to be taken intoaccount in calculating the score Sj of the destination intersection Ij.

For example, this distance Dj is calculated from geographicalinformation from a digital road map preloaded into a standard onboardnavigation device and correlated with position information for thetarget vehicle C, which is updated regularly.

The score Sj of the candidate destination intersection Ij is calculatedin a third step E38.

This score Sj includes a first component f(T_(ij)) linked to the roadtraffic density (T_(ij)) and a second component g(D_(j)) linked to thedistance (D_(j)), these two components being referred to as the “trafficscore” and the “distance score”, respectively.

The score Sj of the candidate destination intersection Ij is calculatedfrom the following formula (equation 1):Sj=α×f(T _(ij))+β×g(D _(j)), where

T_(ij) represents the road traffic density on the traffic routeconnecting the current intersection Ii to the candidate destinationintersection Ij;

-   -   D_(j) represents the distance along the curve of the routing        path connecting the candidate destination intersection Ij to the        target vehicle C; and    -   α and β are correction factors for assigning a specific weight        to each of the two components of the score Sj (for example, if        greater importance is to be given to the road traffic density        T_(ij) than to the distance D_(j), values of α and of β are        chosen such that α>β).

The traffic score f(T_(ij)) is obtained by comparing the current numberX of vehicles moving between the current intersection Ii and thedestination intersection Ij to a reference minimum number N of vehiclesenabling continuous transmission of data packets. This reference numbercorresponds to N vehicles with exactly the same radio range regularlydistributed between the current intersection Ii and the destinationintersection Ij so that two successive vehicles are separated by adistance equal to twice the radius of the radio range of a vehicle.

Accordingly, the reference number N can be defined as follows:

${N = {{Int}\left\{ \frac{D_{ij}}{2 \times R} \right\}}},{where}$

D_(ij) represents the distance between the current intersection Ii andthe destination intersection Ij;

-   -   R represents the radius of the radio range of a vehicle (assumed        to be identical for all vehicles); and    -   Int { } is the function that determines the integer part of its        argument.

Different values of the traffic score are defined that each correspondto a traffic density level, as shown in Table 1 below. According to thistable, if the number X of vehicles is between N and 2.N, the trafficscore assigned is 0.6, which corresponds to a moderately-high trafficdensity.

TABLE 1 Number of vehicles (X) Traffic density Traffic score 0 ≦ X < N/4very low 0.0 N/4 ≦ X < N/2 low 0.2 N/2 ≦ X < N moderate 0.4 N ≦ X < 2.Nmoderately high 0.6 2.N ≦ X ≦ 3.N high 0.8 X ≧ 3.N very high 1

The distance score is determined from the following formula:

${g\left( D_{j} \right)} = {1 - \frac{D_{j}}{D_{i}}}$(equation 2), such that −1≦g(Dj)≦1, where Di is the distance between thecurrent intersection Ii and the target vehicle C.

Accordingly, if the candidate intersection Ij is nearer the targetvehicle than the current intersection Ii (in other words if Dj<Di), apositive distance score is obtained. In contrast, if the candidateintersection Ij is farther from the target vehicle C (i.e. if Dj>Di), anegative distance score is obtained, which is a disincentive to choosingthat intersection.

After the score Sj has been calculated for each destination intersectionIj, the destination intersection Ij with the maximum score Sj isselected in a selection step E40.

The destination intersection selected is the intersection that isgeographically nearest the target vehicle C and has the highest roadtraffic level (or density).

One non-limiting example of selecting a destination intersection in aparticular configuration of an urban intervehicular ad hoc network isdescribed below with reference to FIGS. 3 and 4.

FIG. 4 is a diagram representing a road network 10 similar to that ofFIG. 1, comprising traffic routes L1, L2, L3, C1, C2, C3 definingintersections Ii, I10, I20, I30 between them. The entities representedon these traffic routes are vehicles that form an ad hoc network andforward a data packet P between a source vehicle S and a target vehicleC.

The source vehicle S reaching an intersection Ii is considered to beseeking to transmit the data packet P to the target vehicle C. A carriervehicle of the data packet P to be forwarded could obviously beconsidered in an equivalent way.

For this purpose, the source vehicle S must choose a routing directionfor the packet P by selecting a destination intersection.

In the determination step E32 already described, the source vehicle Sdetects three destination intersections which at the time are candidatedestination intersections for routing the data packet P: a firstintersection I10, a second intersection I20, and a third intersectionI30 (see FIG. 4).

To determine to which destination intersection the packet P should berouted, the source vehicle S calculates the scores Sj of these threeintersections in the calculation phase P1 described above.

The respective scores of the first, second, and third candidatedestination intersections I10, I20, I30 are S1, S2, S3.

As shown in FIG. 4, the traffic conditions in this example are such thatthe road traffic Ti2 (shaded area in FIG. 4) on the portion of thetraffic route L2 connecting the current intersection Ii to the seconddestination intersection I20 is denser than the road traffic Ti3 on theportion of the traffic route C1 connecting the current intersection Iito the third destination intersection I30, which itself is denser thatthe road traffic Ti2 on the portion of the traffic route C1 connectingthe current intersection Ii to the first destination intersection I10.In other words (not indicated in FIG. 4): Ti2>Ti3>Ti1.

As shown in FIG. 4, the second destination intersection I20 is thedestination intersection nearest the target vehicle C. The distance D2along the curve between the second destination intersection I20 and thetarget vehicle C is less than the distance D3 along the curve betweenthe third intersection I30 and the target vehicle C, which itself isless than the distance D1 along the curve between the first destinationintersection I10 and the target vehicle C. In other words: D2<D3<D1.

Given that it is nearest the target vehicle C and has the highesttraffic density, the second destination I20 therefore obtains thehighest score S2 (S2>S3>S1).

It is therefore to this intersection I20 that the data packet P isrouted via numerous vehicles moving toward that intersection.

An example of forwarding a data packet P in accordance with oneparticular embodiment of the invention via vehicles located between twosuccessive intersections is described below with reference to FIGS. 5Aand 5B.

FIG. 5A represents a traffic route V between two consecutiveintersections I_(n) and I_(n+1) at a given time N₁.

FIG. 5B shows the same traffic route V as FIG. 5A at a later time N₂such that N₂>N₁. This new configuration is obtained by the FIG. 2prediction step E9, which estimates the current position of the vehiclestraveling on this traffic route from information on their position andspeed stored and kept up to date by each mobile node.

In this example, a carrier vehicle R0 of the data packet P moving on theroute V toward the destination intersection I_(n+1) is considered to beseeking to forward that packet towards the destination intersectionI_(n+1). In accordance with the invention, the carrier vehicle R0 usesfor this purpose vehicles moving on the route V to the destinationintersection I_(n+1).

As shown in FIGS. 5A and 5B, four vehicles (R1, R2, R3, R4) are withinthe radio range K of the carrier vehicle R0.

The carrier vehicle R0 is therefore able to communicate with one or moreof these vehicles to route the data packet P toward the destinationintersection I_(n+1).

In FIG. 5A it is assumed, for example, that at time N₁:

-   -   the first and second vehicles R1 and R2 are moving on the        traffic route V in the same direction as the carrier vehicle R0,        i.e. toward the destination intersection I_(n+1);    -   the first vehicle R1 is moving faster than the second vehicle        R2; and    -   the third and fourth vehicles R3 and R4 are moving on the        traffic route V in the opposite direction to the carrier vehicle        R0.

According to the routing protocol of the invention, the carrier vehicleR0 transfers the data packet P to the first vehicle R1 because itestimates that, at the time N₂, the first vehicle R1 will have reached aposition nearest the destination intersection I_(n+1), as shown in FIG.5B.

However, note that without this prediction step, which takes account inparticular of the speed and the direction of movement of the neighborvehicles, the carrier vehicle R0 would have chosen to transfer itspacket P to the fourth vehicle R4, given that at time N₁ the vehiclenearest the destination intersection I_(n+1), is the fourth vehicle R4.

The prediction step of the invention advantageously takes intoconsideration the fact that the fourth vehicle R4 and the third vehicleR3 are moving in the opposite direction to the carrier vehicle R0.Because of this, the carrier vehicle R0 excludes these two vehicles fromconsideration for forwarding the packet P.

An example of implementing the recovery strategy in one particularembodiment of the invention is described below with reference to FIGS.6A and 6B.

The FIG. 6A example illustrates the situation where, at time N₁, acarrier vehicle R0 seeking to forward a packet P finds no movingneighbor vehicle between the carrier vehicle R0 and the destinationintersection I_(n+1).

In this situation, a recovery solution conforming to the method of theinvention is used to prevent the packet P getting stuck in a “localminimum” situation, the carrier vehicle R0 being the vehicle nearest thedestination intersection I_(n+1).

This solution entails the carrier vehicle R0 carrying the packet P asfar as the destination intersection I_(n+1). This strategy can beenvisaged provided that the carrier vehicle R0 is not too far from thedestination intersection I_(n+1) (step E19 in FIG. 2).

The FIG. 6B example shows the use of a variant of the above recoverysolution whereby the carrier vehicle R0 carries the packet P until anappropriate vehicle enters its radio range and the packet P cantherefore be transferred to it.

This variant is used in particular if the carrier vehicle R0 is notsufficiently near the destination intersection I_(n+1) (step 21 in FIG.2).

1. A method of routing one or more data packets between a source nodeand a target node in an ad hoc network comprising a plurality of mobilenodes moving on traffic routes of a particular geographical networkforming between them a plurality of intersections, wherein the methodcomprises: a step of a carrier node of a data packet to be routed to thetarget node selecting a destination intersection from candidate neighborintersections as a function of traffic conditions; a step of seeking oneor more neighbor mobile nodes of the carrier node of the data packetnearer the selected destination intersection than the carrier node; andif one or more neighbor mobile nodes is found, a step of transferringthe data packet from the carrier node of the data packet to saidneighbor node that has been found so as to route said data packet towardthe selected destination intersection, wherein a step of selecting a newdestination intersection is executed if the carrier node of the datapacket is located at the selected destination intersection, referred toas a current intersection, and wherein during the selection step thecarrier node of the data packet selects a destination intersection fromthe candidate neighbor intersections as a function of one or more realtime traffic conditions on a traffic route connecting the respectivecandidate intersection to the current intersection.
 2. The methodaccording to claim 1, wherein said carrier node selects the destinationintersection from the candidate neighbor intersections as a function ofthe proximity of the candidate neighbor intersection to the target node.3. The method according to claim 1, wherein if no neighbor mobile nodeis found in the search step, the carrier node carries the packet untilit detects a new neighbor node.
 4. A computer program stored on acomputer memory and executing on a processor, which when used on acommunications terminal causes the processor to execute the steps of therouting method according to claim
 1. 5. A non-transitorycomputer-readable storage medium storing a computer program comprisinginstructions for executing the steps of the routing method according toclaim
 1. 6. A method of routing one or more data packets between asource node and a target node in an ad hoc network comprising aplurality of mobile nodes moving on traffic routes of a particulargeographical network forming between them a plurality of intersections,wherein the method comprises: a step of a carrier node of a data packetto be routed to the target node selecting a destination intersectionfrom candidate neighbor intersections as a function of trafficconditions; a step of seeking one or more neighbor mobile nodes of thecarrier node of the data packet nearer the selected destinationintersection than the carrier node; and if one or more neighbor mobilenodes is found, a step of transferring the data packet from the carriernode of the data packet to said neighbor node that has been found so asto route said data packet toward the selected destination intersection,wherein each node maintains measurements relating to the position, speedand direction of movement of its neighbor nodes and the step of seekingone or more neighbor mobile nodes of the carrier node of the data packetnearer the selected destination intersection than the carrier nodeincludes a prediction step in which the current position of a neighbornode is calculated at a current time from the measurements.
 7. A methodof routing one or more data packets between a source node and a targetnode in an ad hoc network comprising a plurality of mobile nodes movingon traffic routes of a particular geographical network forming betweenthem a plurality of intersections, wherein the method comprises: a stepof a carrier node of a data packet to be routed to the target nodeselecting a destination intersection from candidate neighborintersections as a function of traffic conditions; a step of seeking oneor more neighbor mobile nodes of the carrier node of the data packetnearer the selected destination intersection than the carrier node; andif one or more neighbor mobile nodes is found, a step of transferringthe data packet from the carrier node of the data packet to saidneighbor node that has been found so as to route said data packet towardthe selected destination intersection, wherein said carrier node selectsfrom the candidate neighbor intersections the destination intersectionhaving a maximum score (Sj), the maximum score being calculated usingthe following formula:Sj=α×f(T _(ij))+β×g(D _(j)), where T_(ij) represents the traffic densityof nodes moving between a current intersection and the destinationintersection; D_(j) represents the distance along the curve of therouting path that connects the destination intersection to the targetnode; α and β represent correction factors; f is a density function ofthe road traffic such that 0≦f(T_(ij))≦1; and${{g\left( D_{j} \right)} = {1 - \frac{D_{j}}{D_{i}}}},$ such that −1g(Dj)≦1, where Di is the distance between the current intersection andthe target node.
 8. A communications terminal adapted to be used by amobile node of an ad hoc network for routing data packets to a targetnode of the ad hoc network, said ad hoc network comprising a pluralityof mobile nodes moving on traffic routes of a particular geographicalnetwork forming between them a plurality of intersections, wherein saidterminal comprises: means for selecting a destination intersection fromneighbor intersections as a function of traffic conditions; means forseeking a neighbor mobile node nearer the selected destinationintersection than said mobile node; and means for transferring the datapackets to the neighbor mobile node that has been found if the result ofthe search is positive, wherein a new destination intersection isselected if the mobile node of the data packet is located at theselected destination intersection, referred to as the currentintersection, and wherein during the selection of the new destinationintersection, the mobile node of the data packet selects a destinationintersection from the candidate neighbor intersections as a function ofone or more real time traffic conditions on a traffic route connectingthe respective candidate neighbor intersection to the currentintersection.
 9. A wireless communications system comprising mobilenodes interconnected in an ad hoc structure and moving on traffic routesof a particular geographical network forming between them intersections,wherein said nodes are equipped with a communications terminal accordingto claim 8.